1. Field of the Invention
This invention relates to a process for removal of undesirable components from a hydrocarbon gas stream and in particular to removal of acid gas components and polymer precursors from cracked gas in an ethylene plant upstream of a caustic tower.
2. Description of the Related Art
In the petroleum and petrochemical industries it is common to remove acid gas components, such as carbon dioxide (CO.sub.2) and hydrogen sulfide (H.sub.2 S), from hydrocarbon gas streams by intimate contact with an aqueous solution of a base such as sodium hydroxide (NaOH), which is a caustic solution. By reaction with the caustic of the caustic solution, i.e. NaOH, acid gas components such as hydrogen sulfide and carbon dioxide are converted into sodium sulfide (Na.sub.2 S), sodium hydrosulfide (NaHS), sodium carbonate (Na.sub.2 CO.sub.3) and sodium bicarbonate (NaHCO.sub.3) which are absorbed into the caustic solution and, thus, removed from the hydrocarbon gas stream.
One type of petrochemical operation wherein an aqueous sodium hydroxide solution is almost invariably used for gas scrubbing is in an ethylene production unit or plant. In an ethylene plant a saturated aliphatic hydrocarbon feed, such as ethane, propane, or higher molecular weight hydrocarbon mixtures such as naphtha, atmospheric and/or vacuum gas oils, and the like, is heated at high temperatures in the presence of steam to crack the saturated hydrocarbon molecules down to lower molecular weight unsaturated hydrocarbons such as ethylene predominately, followed by propylene, and then various quantities of C.sub.4, C.sub.5 and C.sub.6 mono- and diolefinic hydrocarbons, with a lesser quantity of C.sub.7 and higher weight saturated and unsaturated aliphatic, alicyclic and aromatic hydrocarbon.
During steam cracking any sulfur containing compounds added and/or present in the hydrocarbon feed stream are converted into hydrogen sulfide and/or organically bound sulfur compounds, and also, a content of carbon dioxide is generated by a water-gas shift reaction. The resultant gas mixture from steam cracking is then quenched from a temperature of about 700-1100.degree. to a lower temperature of about 35 to 40.degree. C. whereupon the major portion of its water and C.sub.7+ hydrocarbon content is condensed and separated from the gas mixture. After quenching, the remaining constituents of the gas mixture are conditioned by various steps of gas compression and refrigerative cooling to prepare it for cryogenic distillation whereby its ethylene, propylene and butenes contents will ultimately be recovered in essentially pure form for ultimate use as monomers in the production of various polymers, such as polyethylene, ethylene copolymers, polypropylene and the like.
One step required to properly condition the gas mixture prior to its cryogenic distillation is to scrub the cracked gas essentially free of any acid gas components, such as hydrogen sulfide and carbon dioxide. This is accomplished at some interstage location of a multi-stage gas compression system and, on occasion post-compression, wherein the cracked gas stream is at a pressure of from about 10 to about 20 atmospheres (atm) by contacting the compressed gas stream with an aqueous sodium hydroxide solution by countercurrent contact in a gas-liquid contact vessel often referred to as an "absorber," "scrubber" or "caustic tower." After such gas scrubbing contact the aqueous sodium hydroxide solution which is discharged from the bottom of this tower contains, in addition to some unreacted sodium hydroxide, the sodium sulfide, sodium hydrosulfide, sodium carbonate and sodium bicarbonate that results from the removal of acid gas compounds from the so scrubbed gas stream. To prevent a build-up of the concentration of these components in the caustic tower and to provide for hydraulic room to add a quantity of fresh higher strength caustic solution to the caustic tower to make up for the consumption of caustic therein, a quantity of this weak or "tower spent" caustic solution is bled away from being recirculated back to the tower. However, to maintain a proper liquid volume of caustic solution circulation within the tower, a portion of this weak or "tower spent" caustic solution is recirculated back to the tower. That quantity of the weak or "tower spent" caustic solution bled away from the tower has been referred to in this art as "spent caustic." Such tower spent caustic has to be conditioned by further processing steps in a spent caustic treatment unit to condition it for an environmental sound disposal.
The caustic tower can be an important factor in determining the production capacity of an ethylene production unit because it is necessary to remove acid gas components so that the scrubbed hydrocarbon gas has a minimal, acceptable level of these components. Further, since all of the cracked gas must pass through the caustic tower, if the caustic tower reaches its capacity limit, then the caustic tower can establish an equipment limit for ethylene production. Thus, the capacity of the caustic tower can be an important factor in determining the capacity of the ethylene production unit, regardless whether the ethylene production unit is being designed for grassroots construction or is an already existing facility.
The capacity of a caustic tower can be, and frequently is, adversely affected by polymer formation. The cracked gas contains highly reactive carbonyls and diolefins, which can form polymers, and thus are of concern throughout the plant, but particularly in the caustic tower. The highly reactive carbonyls and diolefins, and possibly other compounds, react or polymerize to form polymers which coat, foul and plug the internals of equipment, such as the caustic tower, which reduces the equipment's efficiency and capacity and, at times, necessitates a shutdown of the equipment for cleaning. Polymer formation in the caustic tower thus reduces its capacity both by reducing its operating efficiency and by necessitating the shutdown of the caustic tower for cleaning and removing deposits of polymeric material.
In summary, an ethylene plant is financially and economically very important, and it is typically advantageous to maximize its production rate. However, the caustic tower in the ethylene plant can limit the production rate. Such a limitation is very costly, and the limitation can be due to the capacity of the caustic tower to remove acid gas components from the cracked gas. The capacity of the caustic tower is at least limited by its mechanical and process design, but the capacity can be further limited by deposits of polymeric material in the caustic tower.